This invention relates to fiber reinforced thermoplastic materials and to methods for producing such materials.
Fiber-reinforced plastic structures have been used for many years with increasing success because of their high strength, light weight and ease of fabrication compared to the wood or metal structures which they replace. Fibers such as glass, carbon and aramid are popular as reinforcement, and thermosetting resins such as polyester, phenolic and epoxy are common polymeric matrices.
Polymeric materials reinforced with continuous filaments are used as precursors for highly-stressed parts such as aerospace components requiring the highest possible strength with the lowest possible weight. Non-uniformity of the materials comprising such parts requires that the parts be over-constructed so that the weakest will surpass the service requirements. More uniform precursor materials would yield parts having less variation in properties and would permit constructing such parts more efficiently to design criteria.
The most important requirements are that each reinforcing fiber be coated on all surfaces by the polymeric matrix, that the matrix be free of voids and that the fibers be distributed as uniformly as possible throughout the matrix. Coating of the fibers has been accomplished in the past by using low-viscosity thermosetting materials or solutions of thermosetting or thermoplastic material, the low viscosity and surface tension aiding the penetration of the matrix materials through a bundle of reinforcing fibers so that substantially all fibers are completely coated. However, low viscosity materials have the drawback of exuding from the fibrous bundle and accumulating to form resin-rich areas, particularly when the fibers are under tension while drying or setting or when gravity acts on a horizontal bundle. Solution coating has the added drawback of leaving voids or resin-poor zones after the solvent evaporates. When groups of non-uniformly coated fibers are made into a test bar and stressed to failure, the failure tends to initiate at resin-rich zones or at voids.
Although thermosetting polymer matrices are widely used, they require a substantial time to harden under heat and pressure and so are not suitable for production of parts at high speeds. The fiber/matrix materials or so-called pre-preg materials must be refrigerated to extend their shelf life. Furthermore, incorrectly-formed parts or trimmed excess cannot be recovered and re-used.
Thermoplastic polymer matrices are potentially suitable for high-speed production of parts because they can be preheated to forming temperature and pressed only long enough to consolidate the materials and cool the matrix to a temperature at which the part may be removed from the mold without distorting. In addition, the cost of waste is greatly reduced because incorrectly-formed parts may be re-shaped, and scrap may be recovered and re-used.
However, thermoplastic matrices are quite difficult to apply uniformly to filament bundles. At temperatures above their melting points, such materials have high viscosity, and they degrade (decompose) rapidly if they are heated excessively in an attempt to lower the viscosity. Using low-viscosity thermoplastic polymer materials results in low-strength parts, whereas the highest possible matrix strength and toughness is usually desired. Such properties are characteristic of high molecular weight polymeric materials having long molecular chains and high melt viscosity.
According to one known technique for making fiber-reinforced thermoplastic polymer composites, a sheet of continuous parallel reinforcing filaments is placed between two thermoplastic films and heat and pressure is used to force melted thermoplastic between the filaments to completely coat all sides of the filaments. The thickness of the films is adjusted to provide the desired ratio of reinforcing filaments to matrix polymer. When a single spread layer of filaments is used, the goal can be accomplished reasonably well, but many layers are needed to make articles of practical thickness. When multiple layers of filaments are used to make a thicker array at a more reasonable cost, the pressure compacts the dry filaments, closing any gaps between them and preventing the thermoplastic polymeric material from penetrating to the center of the mass. The more layers of filaments are present, the more likely the center filaments are to have insufficient resin coating, and the surface layers are to be resin-rich. Thermoplastic resins have high coefficients of thermal expansion, shrinking as they cool, so that resin-rich zones result in residual thermal stresses and stress concentrations in a composite article that can initiate premature failure.